Linetype, colors, and styles

Each gnuplot terminal type provides a set of distinct "linetypes". These may
differ in color, in thickness, in dot/dash pattern, or in some combination of
color and dot/dash. The default linetypes for a particular terminal can be
previewed by issuing the test command after setting the terminal type.
The pre-defined colors and dot/dash patterns are not guaranteed to be
consistent for all terminal types, but all terminals use the special linetype
-1 to mean a solid line in the primary foreground color (normally black).
By default, successive functions or datafiles plotted by a single command will
be assigned successive linetypes. You can override this default by specifying
a particular linetype for any function, datafile, or plot element.

For terminals that support dot/dash patterns, each default linetype has both
a dot-dash pattern and a default color. However, you can override the default
color by using the keyword linecolor, abbreviated lc. For example, the
postscript terminal provides a dashed blue line as linetype 3. The plot
commands below use this same dash pattern for three plots, one in blue (the
default), another in red (the default for linetype 1), and a third in gold.

Lines can have additional properties such as linewidth. You can associate
these various properties, as well as equivalent properties for point symbols,
into user-defined "line styles" using the command set style line. Once
you have defined a linestyle, you can use it in a plot command to control
the appearance of one or more plot elements.